US President Donald Trump toured the Forbidden City with Chinese leader Xi Jinping on Wednesday as he began the crucial leg of an Asian tour intended to build a global front against North Korea's nuclear threats.

After warning North Korea's "cruel dictatorship" against testing the United States during a speech in Seoul, Trump and First Lady Melania Trump were met by Xi and his wife Peng Liyuan for tea at the former imperial palace.

But the genial gathering will be followed on Thursday by a full day of thorny talks, with Trump looking to prod Xi into doing more to squeeze North Korea economically and to address China's massive trade surplus with the United States.

The Trump administration sees Beijing as the key to controlling Pyongyang, which depends on China for its economic survival and for 90 percent of its trade.

Earlier, Trump congratulated Xi on his reappointment as China's Communist Party chief, tweeting: "I very much look forward to meeting with President Xi who is just off his great political victory."

Trump's use of the term "political victory" for the outcome of last month's Communist Party congress was seen by analysts as an attempt to conciliate Xi before tough talks on trade and North Korea.

"He's laying it on thick to put Xi in a good mood because he will have unpleasant things to tell him," said Jean-Pierre Cabestan, China politics specialist at Hong Kong Baptist University.

Xi has prepared an extravagant "state visit-plus" for Trump, with opera and a lavish banquet, and the US leader has brought a business delegation along for the ride.

Deals are expected to be signed, though they may not be enough to allay US concerns about China's massive trade surplus, which narrowed in October but remained high at a monthly $26.6 billion.

- North Korean cult -

Hours earlier in an address to the South Korean parliament, Trump painted a dark picture of Pyongyang as an oppressive, despotic regime.

"North Korea is a country ruled as a cult," the US leader declared.

"At the centre of this military cult is a deranged belief in the leader's destiny to rule as parent protector over a conquered Korean peninsula and an enslaved Korean people."

South Korean lawmakers applauded as the US president, whose tour of Asia has been dominated by the nuclear-armed North, vowed not to be intimidated and warned Pyongyang it should not test American resolve.

The North carried out its sixth, and most powerful, nuclear test in September, and has fired dozens of missiles in recent months.

Two have overflown Japan, and Pyongyang says it can mount a nuclear warhead on a rocket to bring the US mainland within range.

"We will not permit America or our allies to be blackmailed or attacked," Trump said.

Trump gave a preview of what he will ask Beijing to do regarding North Korea.

A senior White House official said China has done "much more that it's ever done in the past" but it could try harder to curb trade at the border with North Korea.

"There are still some financial links that exist that should not under those (UN) resolutions... We're going to work closely with the Chinese to identify that activity and end it," the official said.

Trump will also decide at the end of his Asian tour next Monday whether to re-designate North Korea as a "state-sponsor of terrorism", the White House said.

- DMZ no-go -

In South Korea Trump had to abandon a surprise visit to the Demilitarized Zone (DMZ) that divides the two Koreas because of bad weather, leaving him "pretty frustrated" according to the White House.

South Korean President Moon Jae-In, who had flown earlier before fog closed in, was left waiting for him at the border, which bristles with electric fences, minefields and anti-tank barriers.

In his speech Trump described the DMZ as "the line that today divides the oppressed and the free", where "the flourishing ends, and the prison state of North Korea sadly begins".

The Kim dynasty has ruled the impoverished, isolated North with an iron fist and pervasive personality cult for three generations, showing no tolerance for political dissent.

The regime has for decades been criticised for a range of rights abuses including torture, rape and execution of perceived critics or those trying to flee the country.

It is also known to operate prison camps where hundreds of thousands languish under forced labour, and its 25 million people are barred from contact with the outside world such as foreign television or internet access.

But Trump made overtures to leader Kim Jong-Un, who has overseen rapid advances in its weapons technology.

In what he said was a direct message to the young leader, he told him: "North Korea is not the paradise your grandfather envisioned. It is a hell that no person deserves."

"Yet despite every crime you have committed against God and man, we will offer a path towards a much better future."

It would have to begin, though, with the North stopping ballistic missile development, Trump said, and "complete verifiable and total denuclearisation".